GRK2 Kinases in the Primary Cilium Initiate SMOOTHENED-PKA Signaling in the Hedgehog Cascade.

During Hedgehog (Hh) signal transduction in development and disease, the atypical G protein-coupled receptor (GPCR) SMOOTHENED (SMO) communicates with GLI transcription factors by binding the protein kinase A catalytic subunit (PKA-C) and physically blocking its enzymatic activity. Here we show that GPCR kinase 2 (GRK2) orchestrates this process during endogenous Hh pathway activation in the primary cilium. Upon SMO activation, GRK2 rapidly relocalizes from the ciliary base to the shaft, triggering SMO phosphorylation and PKA-C interaction. Reconstitution studies reveal that GRK2 phosphorylation enables active SMO to bind PKA-C directly. Lastly, the SMO-GRK2-PKA pathway underlies Hh signal transduction in a range of cellular and in vivo models. Thus, GRK2 phosphorylation of ciliary SMO, and the ensuing PKA-C binding and inactivation, are critical initiating events for the intracellular steps in Hh signaling. More broadly, our study suggests an expanded role for GRKs in enabling direct GPCR interactions with diverse intracellular effectors.

Cytoskeletal regulation of a transcription factor by DNA mimicry via coiled-coil interactions.

A long-established strategy for transcription regulation is the tethering of transcription factors to cellular membranes. By contrast, the principal effectors of Hedgehog signalling, the GLI transcription factors, are regulated by microtubules in the primary cilium and the cytoplasm. How GLI is tethered to microtubules remains unclear. Here, we uncover DNA mimicry by the ciliary kinesin KIF7 as a mechanism for the recruitment of GLI to microtubules, wherein the coiled-coil dimerization domain of KIF7, characterized by its striking shape, size and charge similarity to DNA, forms a complex with the DNA-binding zinc fingers in GLI, thus revealing a mode of tethering a DNA-binding protein to the cytoskeleton. GLI increases KIF7 microtubule affinity and consequently modulates the localization of both proteins to microtubules and the cilium tip. Thus, the kinesin-microtubule system is not a passive GLI tether but a regulatable platform tuned by the kinesin-transcription factor interaction. We retooled this coiled-coil-based GLI-KIF7 interaction to inhibit the nuclear and cilium localization of GLI. This strategy can potentially be exploited to downregulate erroneously activated GLI in human cancers.

Interplay between the Kinesin and Tubulin Mechanochemical Cycles Underlies Microtubule Tip Tracking by the Non-motile Ciliary Kinesin Kif7.

The correct localization of Hedgehog effectors to the tip of primary cilia is critical for proper signal transduction. The conserved non-motile kinesin Kif7 defines a "cilium-tip compartment" by localizing to the distal ends of axonemal microtubules. How Kif7 recognizes microtubule ends remains unknown. We find that Kif7 preferentially binds GTP-tubulin at microtubule ends over GDP-tubulin in the mature microtubule lattice, and ATP hydrolysis by Kif7 enhances this discrimination. Cryo-electron microscopy (cryo-EM) structures suggest that a rotated microtubule footprint and conformational changes in the ATP-binding pocket underlie Kif7's atypical microtubule-binding properties. Finally, Kif7 not only recognizes but also stabilizes a GTP-form of tubulin to promote its own microtubule-end localization. Thus, unlike the characteristic microtubule-regulated ATPase activity of kinesins, Kif7 modulates the tubulin mechanochemical cycle. We propose that the ubiquitous kinesin fold has been repurposed in Kif7 to facilitate organization of a spatially restricted platform for localization of Hedgehog effectors at the cilium tip.

ALIX is recruited temporarily into HIV-1 budding sites at the end of gag assembly.

Polymerization of Gag on the inner leaflet of the plasma membrane drives the assembly of Human Immunodeficiency Virus 1 (HIV-1). Gag recruits components of the endosomal sorting complexes required for transport (ESCRT) to facilitate membrane fission and virion release. ESCRT assembly is initiated by recruitment of ALIX and TSG101/ESCRT-I, which bind directly to the viral Gag protein and then recruit the downstream ESCRT-III and VPS4 factors to complete the budding process. In contrast to previous models, we show that ALIX is recruited transiently at the end of Gag assembly, and that most ALIX molecules are recycled into the cytosol as the virus buds, although a subset remains within the virion. Our experiments imply that ALIX is recruited to the neck of the assembling virion and is mostly recycled after virion release.

Identification of pauses during formation of HIV-1 virus like particles.

HIV Gag polymerizes on the plasma membrane to form virus like particles (VLPs) that have similar diameters to wild-type viruses. We use multicolor, dual-penetration depth, total internal reflection fluorescence microscopy, which corrects for azimuthal movement, to image the assembly of individual VLPs from the time of nucleation to the recruitment of VPS4 (a component of the endosomal sorting complexes required for transport, and which marks the final stage of VLP assembly). Using a mathematical model for assembly and maximum-likelihood comparison of fits both with and without pauses, we detect pauses during Gag polymerization in 60% of VLPs. Pauses range from 2 to 20 min, with an exponentially distributed duration that is independent of cytosolic Gag concentration. VLPs assembled with late domain mutants of Gag (which do not recruit the key endosomal sorting complexes required for transport proteins Alix or TSG101) exhibit similar pause distributions. These pauses indicate that a single rate-limiting event is required for continuation of assembly. We suggest that pauses are either related to incorporation of defects in the hexagonal Gag lattice during VLP assembly or are caused by shortcomings in interactions of Gag with essential and still undefined cellular components during formation of curvature on the plasma membrane.

Protein dynamical transition does not require protein structure.

Terahertz time domain spectroscopy shows that the protein dynamical transition, the rapid increase in protein dynamics occurring at approximately 200 K, needs neither tertiary nor secondary structure. Further, short chain alanine studies find a dynamical transition down to penta-alanine, with no transition observed for di-alanine or tri-alanine. These results reveal the temperature dependence arises strictly from the side-chain interaction with the solvent. The lack of a transition for shorter chain peptides may indicate a qualitative change in this interaction occurs at a specific peptide chain length.